The present invention relates to a controllable inductor comprising at least a tubular core, a main winding surrounding the core and a control winding passing substantially axially through the core.
Such controllable inductors may through the main winding thereof be connected to any electrical circuit, such as an electric power line, so as to provide this circuit with an inductance, for example for extinguishing harmonic currents generated in the circuit. The magnetic permeability of the core and by that inductance of the inductor may then be controlled by modifying an electric control current caused to flow axially through the core in the control winding. By connecting such a controllable inductor in series with a capacitor a so called harmonic filter is obtained, which is already known, for example as described in WO 94/11891 of the applicant and in which the impedance may be controlled to be low for certain frequencies by controlling the inductance of the inductor for fading out harmonic currents having a frequency being a multiple, for example 11, of the fundamental frequency of the network.
Usually an alternating voltage is connected to the main winding, but it would also be possible to connect a direct voltage with an overlapped alternating voltage to the main winding, but in that case the inductor would only have a useful influence upon the alternating voltage part. The control current brought to flow through the control winding is normally a direct current, but it would just as a well be possible to use an alternating current as control current and by controlling such an alternating current control current appropriately even voltage induced in the control winding, which causes harmonic currents in the main winding and losses in the core, may be eliminated.
In controllable inductors already known the main magnetic flux extending substantially axially through the core is closed in the air outside the core and the main winding, so that a so called air reactor is formed. A disadvantage of such an inductor is that it allows a regulation of the inductance within a comparatively narrow interval, most often only by a factor of about 10%. This narrow regulation interval of the inductance of such an inductor greatly limits its field of use, and it may therefore mainly be used as an harmonic filter.
Also other controllable inductors not having any control winding are known, but these may in principle be regarded as fixed, but through intermittently connecting different such inductors to the circuit in question it is possible to provide it with a controllable inductance. In such fixed inductors functioning according to this principle the harmonic or overtone generation will be great with considerable disadvantages resulting therefrom, primarily the need of several filter banks so as to eliminate harmonics generated. Additionally, these inductors have to be controlled by thyristors to be water-cooled and by that controlled through an expensive control equipment.
The object of the present invention is to provide a controllable inductor of the type defined in the introduction, which has a simple construction and by that is inexpensive therefore, the function thereof is reliable and this makes it possible to regulate the inductance within a comparatively wide range for broadening the field of use of such a controllable inductor with respect to the already known inductors discussed above.
This object is according to the invention obtained by providing such a controllable inductor with a yoke of a material having a high magnetic permeability arranged to extend outside the core and the main winding and together with the core form a close loop having at the most small air gaps for a main magnetic flux generated in the core by a current in the main winding and extending substantially axially in the core. The control winding comprises first plates extending substantially axially through the core and being of a material having a good electric conductivity.
Due to the fact that the control winding comprises the first plates it is possible to obtain a control winding at a low cost and most importantly a stable mechanical construction of the inductor. This makes it possible to conduct the control winding in a path outside the main winding, by which it is possible to arrange a yoke closing the main magnetic flux through the core with at the most small air gaps in the loop provided for the main magnetic flux. The controllability its is very high in such an inductor having at the most small air gaps, since the main part of the energy stored will be within material having a low magnetic permeability, contrary to a so called air reactor in which a great part of the energy is stored in the air and, by that, it may not be regulated as easily. In an inductor of the type according to the invention, regulation of the inductance is made possible to a considerably greater extent than in known inductors, such as easily by a factor 5 or more. xe2x80x9cSmall air gapsxe2x80x9d are defined as air gaps being small with respect to the thickness of the wall of the core, so that eddy current losses may be avoided. It would be possible to arrange many small distributed air gaps in the core, when no particularly great controllability of the inductance of the inductor is needed, since in this way iron may be saved and the entire inductor could be less costly. However, the greatest controllability is obtained when the air gap is at a minimum.
A possible field of use for an inductor of this type is a switching in alternating voltage power lines, which have a high capacitance built-in therein, for example cable networks. By an intermediate connection of the inductor an inductance of a desired size may be added and by that the reactance of the power line may be reduced for a more efficient energy transfer through the line.
According to a preferred embodiment of the invention the control winding also comprises second plates extending outside the core and the main winding and being of a material having a good electric conductivity. The plates are electrically connected to the first plates and adapted to together therewith form closed loops for a current flow in the first plates through the core. By such a construction of the control winding a very stable mechanical construction, is achieved so that the function thereof will be constant over time and the device is reliable, and it also becomes simple and cheap to manufacture.
According to another preferred embodiment of the invention, the first plates are arranged in different packages of plates pressed together with large flat surfaces thereof against each other. The packages have one or more plates, and these plate packages have substantially the same cross section. Plates having one thickness may be used for obtaining electric conductors, which are constituted by the plate packages, for the control current through the core having substantially the same cross section, so that substantially just as much heat loss is generated in each conductor and there will be no problem with local superheating.
According to a further development of the embodiment last mentioned, the plate packages have a thickness reduced in the direction of a radius of the core perpendicularly to the large flat surfaces of the first plates towards the centre of the core for obtaining a maximum filling of the inner hollow space of the core. By such a design of the control winding, i.e. a reduction of the thickness of the control plates where these may be made wider in the direction of their large flat surfaces thereof in parallel with a radius of the core, a maximum filling of the inner hollow space of the core and by that a good controllability of the inductor may be obtained.
According to another preferred embodiment of the invention the yoke comprises plates arranged to extend substantially in parallel with each other at each end of the core and being with their large surfaces thereof in parallel with a plane defined by a radius of the core and the axis of the core. The yoke plates have an edge thereof located in the immediate vicinity of the respective core end so as to receive the main magnetic flux from the core without any substantial air gap therebetween. The second control plates are arranged in a space between yoke plates arranged side by side and have substantially the same direction in the room as the yoke plates, so the yoke plates and these control plates form a sandwich construction. This embodiment is very advantageous, since yoke plates extending substantially in parallel with each other without any real disturbance of the second control plates included in the control winding may be brought to cover substantially the entire area through which the main magnetic flux may be expected to cover. Thus no cross flux plates are needed and a cross magnetization of the yoke and eddy current losses caused thereby is avoided. Because the said plates of the control winding are of a material having a low magnetic permeability, i.e. having a high reluctance, the reluctance perpendicular to the yoke plates may in this way be made high, so that the control flux is prevented from going out of the core and into the yoke at the ends of the core. A control flux in the yoke would impair its permeability and result in increased losses.
According to a further preferred embodiment of the invention the second control plates are arranged with their large flat surfaces substantially in parallel with the large flat surfaces of the first control plate, and at least a first control plate of each control plate package is arranged to protrude from the core past the edge of the respective second control plate located closest to the core so as to enter into electric contact establishing bearing thereagainst. A stable closed loop of the control winding may in this way be easily formed.
According to another preferred embodiment of the invention the inductor is intended to be connected to a three-phase alternating current network and it has one core and one main winding for connection to each phase. Such an inductor is particularly advantageous since the voltages induced in the control windings through the alternating main magnetic flux will cancel each other out, so that a generation of harmonics in the network and losses in the core are avoided.
According to another preferred embodiment of the invention the inductor has, for connection to a multiphase alternating-current network, a yoke in common to and closing the main magnetic flux through all the cores and forming main magnetic flux paths between all cores. This is important for keeping the main magnetic flux within the parts having a high magnetic permeability (yoke and core), since the main magnetic flux flowing through a core has to be able to be distributed on the other cores and the sum of the main magnetic flux has in each moment to be zero.
According to another preferred embodiment of the invention the control winding of each core is electrically connected to a control winding for the adjacent core through the second control plates and the control winding of the two cores located outermost is through the control plates also electrically connected to one outer leg each of third control plates, which like the first control plates connect second control plates on one end of the cores to second control plates on the other end of the cores. By arranging such an outer leg with three control plates it may easily be accomplished that a control current is running through all first control plates in all the cores.